The present invention relates to the improvement of the image quality of printers that produce images by an array of printed dots, and, more particularly, to improved image quantity for ink jet printers.
Printers are devices that print images onto a printing medium such as a sheet of paper. Printers of many types are available, and are commonly linked to a computer that supplies the content of the images, in the form of text, characters, or figures, that are to be printed.
A dot printer forms the image by printing a pattern of individual dots on particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small squares or circles in a rectilinear array. The locations are termed "pixels", and the array is termed the "pixel array". Thus, the printing operation can be viewed as the filling of the correct pattern of pixels of the pixel array with a colorant. This type of image formation is similar to that used for photograhic images in newspapers.
An ink jet printer forms small droplets of a colorant such as an ink or a dye that are ejected toward the printing medium. The droplets are ejected from a print head that has an array of nozzles therein. In a familiar type of ink jet printer, the print head is traversed back and forth across the face of the printing medium to print a swath, and the printing medium is incrementally advanced periodically so that the nozzle array can pass over another swath. The electronic driving circuitry of the printer commands the individual nozzles to eject small droplets of colorant at the appropriate points to fill the proper pixels of the desired pixel array, and also controls the traversing and advance functions of the printer.
Since the image is built up incrementally from thousands of dots, the quality of the image is ultimately dependent upon the quality of the individual dots and their interrelationship with each other. Desirably, each dot would be a generally circular, well formed spot of dried colorant, and each dot would fill its proper pixel to the pixel boundaries without extending into neighboring pixels.
A number of factors can prevent the formation of such a desirable array of dots. One problem is the bleeding of droplets into each other during printing. Thus, if two neighboring pixels are to be printed, two droplets of colorant are deposited as the print head passes. The volume of colorant may be so large that the droplets flow or bleed into each other before they can dry. A portion of the large volume of liquid may flow out of the intended pixels into other areas by the wicking action of the paper. The result may be run-together and improperly formed pixel dots. The problem is compounded when there are multiple pixels formed adjacent each other before drying occurs, so that the printing medium becomes saturated with the pool of colorant.
While present in black-and-white printing, the pixel bleeding problem is of particular concern in color printing. Since secondary colors are formed by the overprinting of primary colors, bleeding from one pixel to another can be especially severe at boundaries between secondary colors. Even though the individual droplets are small, the color bleeding may be readily discerned by the eye that views the completed image.
To reduce color bleeding, special highly absorbent papers have been developed that cause the colorant to dry before substantial movement by wicking or flow can occur. While effective, such an approach has the drawback that the use of untreated or plain paper does not yield the best performance of the printer. In another approach to solving the color bleed problem for color images that is widely used, the various color pens can be offset slightly from each other along the traversing direction, so that a first ejected droplet of colorant of one color has a chance to dry before the next droplet is deposited. The offset of the pens in the traversing direction cannot normally be large enough to permit complete drying, so that some color bleed problem remains.
In yet another approach, the pixels are printed in a first checkerboard pattern and then an overlapping checkerboard pattern displaced by one pixel spacing from the first pattern to fill the entire image. The color bleed problem between the colorant dots that are adjacent in the traverse direction or the advance direction is reduced, but in each case a diagonally oriented pixel is printed on the same traverse, so that colorant bleed may occur along that diagonal. Printer throughput is halved using this approach, unless scan speed is increased, which increases the size and cost of the printer.
In yet another approach set forth in U.S. Pat. No. 4,540,996, the nozzles are spaced at a multiple of the pixel spacing, and the print head is advanced by another multiple of the pixel spacing. Relief is provided as to bleeding of the different colors from pixel to pixel in the advance direction, but there is no indication of a reduction in colorant bleeding from pixel to pixel in the traversing direction.
There exists a continuing need for an improved dot printing strategy for black-and-white and color dot printers such as ink jet printers. The present invention fulfills this need, and further provides related advantages.